The catalytic hydrogenation of hydrocarbon feedstock, such as untreated gas-oil, is conventionally conducted in a hydrotreater. Such a hydrotreater is commonly a vertically extending tower whose internal chamber is sub-divided into a plurality of bed sections by horizontally extending grid structures termed `interbed assemblies`. Each bed section is packed with particulate catalyst, typically in the form of pellets that might have a length in the order of 15 mm and a diameter in the order of 3 mm.
An interbed assembly commonly comprises a steel rod grid underlain by a splash tray and a bubble cap tray. The assembly is designed to support a bed of catalyst whilst permitting the fluids in the reactor to pass therethrough.
Each interbed assembly commonly comprises a plurality of vertically disposed, open ended drain tubes which extend through the assembly. These tubes are normally filled with a column of steel balls whereby the catalyst beds are isolated one from another and yet fluids can easily pass from one section to another.
When the catalyst is spent, it must be removed from the tower to be either discarded or regenerated. This is commonly effected by a combination of two sequentially practised procedures. In accordance with the first procedure, the upper bed sections are first emptied, by inserting a vacuum hose through an aperture in the top of tower and sucking the catalyst upwardly out of the tower. If the second bed section is to be cleaned in the same way, the steel balls are removed from the drain tubes of the uppermost interbed assembly and the vacuum hose is extended through one of the drain tubes into the next section. Once the uppermost bed sections of the tower have been cleaned out in this way, an outlet is opened in the base of the tower. The remaining catalyst and the balls in the tubes then drain downwardly through the drain tubes and bed sections and are removed through the outlet from the tower.
Now, there is a reason why the uppermost bed or beds are recovered separately from lower beds. The catalyst at the top of the tower, where the hydrocarbon feed is usually introduced, normally is poisoned and exhausted at a faster rate than the catalyst lower down in the tower. Thus, at the end of a run, when the catalyst charge as a whole is no longer working at the desired level of efficiency, it is often desirable to recover the lowermost catalyst separately, as it may be regenerated and re-used, while the same procedure cannot be practiced with success on the uppermost catalyst.
Since the uppermost catalyst would drain with the lowermost catalyst if both were in the tower and the bottom outlet was opened and removal of catalyst therethrough began, it has heretofore been the practise, as far as I know, to first vacuum out the upper beds to be discarded, before beginning to drain the bottom beds.
This sequential technique is time-consuming. By way of example, in a 3-bed, 88 foot tall hydrotreater, having the first interbed assembly 12 feet from the top, the next 39 feet from the first, and the third 36 feet from the second, it required a total of 10 shifts to empty the tower using the sequential system. Of this total, two 12-hour shifts were required to vacuum out the upper bed section and eight 12-hour shifts were required to drain out the two lower bed sections. When both of the upper beds were to be vacuumed and qnly the bottom bed was to be drained, it took nine 12-hour shifts to complete vacuuming and four 12-hour shifts to drain.